A keystone species is a species that exerts an impact on its community that is both strong and disproportionate to its abundance. The keystone analogy refers to the architectural element at the apex of an arch that locks the other pieces into position, and is used colloquially to refer to the supporting element of a larger structure. Paine (1969) originally defined a keystone predator as a species that feeds preferentially on the dominant competitor among its prey species, such that the keystone predator’s feeding prevents the dominant prey from excluding other species, and therefore maintains a higher species diversity in the system than in the keystone’s absence. In the literature of ecological science, keystone predation is now often used to refer to predation on a dominant competitor that, as a consequence, maintains high prey diversity in the system.
Since Paine’s introduction of the term, the keystone concept has been generalized to a widening range of phenomena loosely grouped by the fact that they have important influences on some aspect of ecosystem structure or functioning. In an effort to clarify and standardize the meaning of the keystone species concept, Power et al. (1996) introduced an important practical distinction by defining a keystone species as one whose effect is both “large, and disproportionately large relative to its abundance”. This definition distinguishes keystone species from dominant species--in the latter case, effects are large at least in part because the species is very abundant. For example, in northern boreal forests, spruce has a fundamental and large impact on the structure and function of the ecosystem, largely because it is far and away the most abundant plant species—that is, it is a dominant species. In contrast, in the northeastern Pacific Ocean, sea otters have very strong cascading impacts on the structure and functioning of rocky shore ecosystems despite their relatively low abundance. Their disproportionate impacts stem from their voracious appetite for sea urchins, herbivorous invertebrates whose feeding on kelps in turn determine whether rocky bottoms will become dominated by large kelps (in the absence of urchins) versus by crustose calcified algae (in the presence of urchins). The kelp beds facilitated by sea otter predation on urchins offer both abundant food and physical structure in which fishes and other animals can grow and escape their predators, whereas the crustose algal “pavements” that develop in the absence of sea otters leave the bottom with no structure and little plant biomass to support other animals. Thus, in the northeast Pacific, sea otters are a keystone species because, despite their relatively low abundance, they are largely responsible for maintaining the existence of kelp beds and thus have a fundamental impact on coastal ecosystems.
Although the keystone species concept originally referred specifically to a predator, the definition of Power et al. implies that keystone species can exert their influence through other interactions as well, including competition, mutualism, pollination, dispersal, etc. For example, species that exert a large impact on their ecosystem by modifying habitat have been referred to as “ecosystem engineers”. If an ecosystem engineer performs its function even when it is at low population abundance, it could be considered a keystone species. Beavers are the classic example. Diseases that have strong impacts on their hosts would also be classic examples of keystone species under the broad definition of Power et al., since the disease organisms generally have a very small biomass in the system. For example, an outbreak of rinderpest in the late 19th century in southern Africa had a catastrophic impact on ungulates, and their loss cascaded to cause a fundamental transition in the African landscape, from grassland to woody savannah.
The concept of keystone species has also become an important issue in conservation biology insofar as the loss or decline of keystone species may have far-reaching consequences for the structure and functioning of the ecosystems in which they live. For example, there is concern--and some evidence--that declines of relatively rare top predators in terrestrial ecosystems have allowed populations of herbivores such as white-tailed deer to explode, with strong impacts on the biomass and species composition of terrestrial vegetation.
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